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Abstract

We present an amplification medium for optical parametric chirped-pulse amplification that allows for ultrabroadband gain in a collinear configuration. Our approach is based on aperiodic quasi-phase-matching (QPM). For the first demonstration of this method in a mid-IR optical parametric chirped-pulse amplifier, we chose a QPM grating design with a linear chirp of its associated spatial frequencies. The resulting 7.4-mm-long, aperiodically poled Mg:LiNbO3 amplification crystal has a chirp rate of κ′=−250cm−2 and provides gain over the 800nm bandwidth centered at 3.4μm. We were able to generate pulses as short as 75fs and the pulse energy at the output of the optical parametric amplifier before compression was 1.5μJ. Low thermal load on the amplification medium allows for operation at a high repetition rate, 100kHz in our case, and high average power limited only by the available pump power.

Gain spectrum of the first OPA stage, showing more than 800nm amplification bandwidth when pumped at 1.9GW/cm2. The measurement is limited only by the available seed bandwidth, giving rise to noise in the spectral wings.

The power spectrum of the seed (blue dashed–dotted curve) supports 71fs transform-limited pulses. After the first amplifier (green dashed curve), the spectrum is broadened, supporting 65fs pulses, and further broadened after the second amplifier (red solid curve), yielding spectral bandwidth for 54fs pulses.

Left graph shows the power scaling of OPA1. There is only negligible OPG output. In the right graph. one can clearly see the amplification of the remaining OPG of OPA1. This can be suppressed by the use of a steeper long-pass filter between the two OPA stages. A completely unseeded OPA2 generates less than 2mW OPG.